The Institute of Electrical and Electronics Engineers, Inc. (IEEE) 802.11 standard is a family of standards for wireless local area networks (WLAN) in the unlicensed 2.4 and 5 Gigahertz (GHz) bands. The current 802.11b standard defines various data rates in the 2.4 GHz band, including data rates of 1, 2, 5.5 and 11 Megabits per second (Mbps). The 802.11b standard uses direct sequence spread spectrum (DSSS) with a chip rate of 11 Megahertz (MHz), which is a serial modulation technique. The 802.11a standard defines different and higher data rates of 6, 12, 18, 24, 36 and 54 Mbps in the 5 GHz band. It is noted that systems implemented according to the 802.11a and 802.11b standards are incompatible and will not work together.
A new standard is being proposed, referred to as 802.11g (the “802.11g proposal”), which is a high data rate extension of the 802.11b standard at 2.4 GHz. It is noted that, at the present time, the 802.11g proposal is only a proposal and is not yet a completely defined standard. Several significant technical challenges are presented for the new 802.11g proposal. It is desired that the 802.11g devices be able to communicate at data rates higher than the standard 802.11b rates in the 2.4 GHz band. In some configurations, it is desired that the 802.11b and 802.11g devices be able to coexist in the same WLAN environment or area without significant interference or interruption from each other, regardless of whether the 802.11b and 802.11g devices are able to communicate with each other. It may further be desired that the 802.11g and 802.11b devices be able to communicate with each other, such as at any of the standard 802.11b rates.
A dual packet configuration for wireless communications has been previously disclosed in U.S. patent application entitled, “A Dual Packet Configuration for Wireless Communications”, application Ser. No. 09/586,571 filed on Jun. 2, 2000, which is hereby incorporated by reference in its entirety. This previous system allowed a single-carrier portion and an orthogonal frequency division multiplexing (OFDM) portion to be loosely coupled. Loosely coupled meant that strict control of the transition was not made to make implementations simple by allowing both an existing single-carrier modem and an OFDM modem together with a simple switch between them with a minor conveyance of information between them (e.g., data rate and packet length). In particular, it was not necessary to maintain strict phase, frequency, timing, spectrum (frequency response) and power continuity at the point of transition (although the power step would be reasonably bounded). Consequently, the OFDM system needed to perform an acquisition of its own, separate from the single-carrier acquisition, including re-acquisition of phase, frequency, timing, spectrum (including multi-path) and power (Automatic Gain Control [AGC]). A short OFDM preamble following the single carrier was used in one embodiment to provide reacquisition.
An impairment to wireless communications, including WLANs, is multi-path distortion where multiple echoes (reflections) of a signal arrive at the receiver. Both the single-carrier systems and OFDM systems must include equalizers that are designed to combat this distortion. The single-carrier system designs the equalizer on its preamble and header. In the dual packet configuration, this equalizer information was not reused by the OFDM receiver. Thus, the OFDM portion employed a preamble or header so that the OFDM receiver could reacquire the signal. In particular, the OFDM receiver had to reacquire the power (AGC), carrier frequency, carrier phase, equalizer and timing parameters of the signal.
Interference is a serious problem with WLANs. Many different signal types are starting to proliferate. Systems implemented according to the Bluetooth standard present a major source of interference for 802.11-based systems. The Bluetooth standard defines a low-cost, short-range, frequency-hopping WLAN. Preambles are important for good receiver acquisition. Hence, losing all information when transitioning from single-carrier to multi-carrier is not desirable in the presence of interference.
There are several potential problems with the signal transition, particularly with legacy equipment. The transmitter may experience analog transients (e.g., power, phase, filter delta), power amplifier back-off (e.g. power delta) and power amplifier power feedback change. The receiver may experience AGC perturbation due to power change, AGC perturbation due to spectral change, AGC perturbation due to multi-path effects, loss of channel impulse response (CIR) (multi-path) estimate, loss of carrier phase, loss of carrier frequency, and loss of timing alignment.